BU-803a: Cell Matching and Balancing

A manufacturer cannot predict the exact capacity when the cell comes off the production line, and this is especially true with lead acid and other batteries that involve manual assembly. Even fully automated cell production in clean rooms causes performance differences. As part of quality control, each cell is measured and segregated into categories according to their capacity levels. The high-capacity NiMH and other cells may be reserved for special applications and sold at premium prices; the large mid-range will go to commercial and industrial markets; and the low-grade cells might end up in a consumer product or in a department store. Cycling will not significantly improve the capacity of the low-end cell, and the buyer should be aware of differences in capacity and quality, which often translate into life expectancy.

Cell matching according to capacity is important, especially for industrial batteries, and no perfect match is possible. If slightly off, nickel-based cells adapt to each other after a few charge/discharge cycles similar to the players on a winning sports team. High-quality cells continue to perform longer than the lower-quality counterparts, and fading is more even and controlled. Lower-grade cells, on the other hand, diverge more quickly with use and time, and failures due to cell mismatch are more widespread. Cell mismatch is a common cause of failure in industrial batteries. Manufacturers of professional power tools and medical equipment are careful with the choice of cells to attain good battery reliability and long life.

Let’s look at what happens to a weak cell that is strung together with stronger cells in a pack. The weak cell holds less capacity and is discharged more quickly than their strong brothers. Going empty first causes their strong brothers to overrun their feeble sibling to the point where a high load can push the weak cell into reverse polarity. Nickel-cadmium can tolerate a reverse voltage of minus 0.2V at a few milliamps, but exceeding this will cause a permanent electrical short. On charge, the weak cell reaches full charge first, and then goes into heat-generating overcharge, while the strong brothers still accept charge and stay cool. The weak cell experiences a disadvantage on both charge and discharge; it continues to weaken until giving up the struggle.

The capacity tolerance between cells in an industrial battery should be +/– 2.5 percent. High-voltage packs designed for heavy loads and a wide temperature range should reduce the capacity tolerance further. There is a strong correlation between cell balance and longevity.

Figure 1 illustrates the cycling performance of five aged Li-ion packs as a function of cell match. The cells are connected in a 2P4S arrangement with a center tap, forming two battery sections that in our example are poorly matched. The capacity differences between the two sections are 5, 6, 7 and 12 percent. When cycled, all batteries show large capacity losses over 18 cycles, but the greatest decrease occurs with the pack exhibiting 12 percent capacity mismatch.

Quality Li-ion cells have uniform capacity and low self-discharge when new. Adding cell balancing is beneficial especially as the pack ages and the performance of each cell decreases at its own pace. A problem arises when a cell in a string loses capacity or develops elevated self-discharge. This can be attributed to high-temperature spots in a large battery. Low-quality cells may also be prone to unequal aging. Li-phosphate has higher self-discharge that other Li-ion, and this complicates cell balancing(See BU-802b: What does Elevated Self-discharge do?)

A battery expert once said: “I have not seen a cell balancing circuit that works.” For multi-cell packs, he suggested using quality Li-ion cells that have been factory-sorted on capacity and voltage. This works well for Li-ion packs up to 24V; packs above 24V should have balancing. Most balancing is passive; active balancing is complex and is only used in very large systems.

Passive balancing bleeds high-voltage cells on a resistor during charge in the 70–80 percent SoC curve; active balancing shuttles the extra charge from higher-voltage cells during discharge to those with a lower voltage. Active balancing is the preferred method for EV batteries, but it requires DC-DC converters. The corrected currents are in the mA range only. Applying a heavy load during acceleration, followed by rapid-charging with regenerative braking requires well-tuned cells in a high-voltage battery to attain the anticipated life. For cost reasons, EV batteries use mainly passive balancing.

Single-cell applications in mobile phones and tablets do not need cell balancing. The capacity between cells can vary and each cell is allowed to age on its own terms without causing harm, other than delivering shorter runtimes. The consumer accepts this decrease; it’s part of planned obsolescence in consumer products(See BU-801a: How to Rate Battery Runtime)

All Li-ion cells require a protection circuit that assures that serially connected cells do not exceed 4.25V/cell (most Li-ion) on charge and that disconnect when the weakest cell drops to 2.80V/cell or lower. The discharge disconnect prevents the stronger cells from pushing the depleted cell into reverse polarity. The protection circuit acts like a guardian angel that shields the weaker siblings from being bullied by the stronger peers. This may explain why Li-ion packs for power tools last longer than nickel-based batteries without a protection circuit. The protection circuit also safeguards the battery from excessive load current(See BU-304: Protection circuits)

With use and time, battery cells become mismatched, and this also applies to lead acid. Cells that develop high self-discharge will lead to imbalance and subsequent failure. Manufacturers of golf cars, aerial work platforms, floor scrubbers and other battery-powered vehicles recommend an equalizing charge if the voltage difference between the cells is greater than +/– 0.10V, or if the specific gravity varies more than 10 points (0.010 on the SG scale).

An equalizing charge is a charge on top of a charge that brings all cells to full-charge saturation. This service must be administered with care because excessive charging can harm the battery(See BU: 404: Equalizing Charge) A difference in specific gravity of 40 points poses a performance problem and the cell is considered defective. (A 40-point difference means one cell has an SG of 1.200 and another 1.240.) A charge may temporarily cover the deficiency, but the flaw will likely resurface again after a few hours due to the high self-discharge of the faulty cell.